P163] Dosimetric effects from high density markers for prostate cancer treatments

Abstract

Purpose Implanted high density gold markers are used in radiotherapy of prostate cancer for a more accurate delivery of planned target dose. However, the presence of high density objects inside the prostate causes artefacts in the computed tomography (CT) images from which the dose calculations are based on. The interpretation of the tissues made by the different dose calculation algorithms becomes uncertain and this uncertainty may propagate to the final dose distribution. The aim of this work was to quantify the dosimetric effects from high density markers on dose distributions for prostate cancer treatments using three different algorithms. Methods About 200 patients treated with volumetric modulated arc therapy (VMAT) for prostate cancer were included. Dose calculations (dose to water) were executed using the treatment planning system (TPS) algorithms, anisotropic analytical algorithm (AAA), Acuros XB (AXB) and by an automated Monte Carlo (MC) system based on the EGSnrc code package with modifications. In order to quantify the effect of high density markers, dose distributions for prostate plans (post-operative and with gold markers) were compared. Dose volume histogram (DVH) estimates such as the mean dose to the clinical target volume (CTV), the planning target volume (PTV), D 98 % PTV and D 2 % PTV (dose to xx% of the PTV) were evaluated. Further analysis was performed by modifying the artefact areas in the CT images by setting the material in the PTV to water. Results The mean difference between AAA, AXB and MC estimations of the mean dose to the CTV and PTV was within 0.5% for all cases. Also, mean dose deviations up to 2% were observed for individual plans, especially for plans with gold markers. Dose deviations up to 5%, related to the shape of the DVH, were detected for D 98 % PTV and D 2 % PTV. The size of the CTV showed no impact on dose deviations for prostate plans with gold markers. Conclusions The presence of gold markers increases the variation between CTV and PTV dosimetry parameters obtained by different algorithms. However, no clinically relevant dosimetric effects arise from artefacts caused by high density markers.